Contrasting trends of the ocean CO2 sink and pH in the agulhas current system and the Mozambique basin, south-western Indian ocean (1963–2023)

Abstract

We describe new observations of the oceanic carbonate system in the South-Western Indian Ocean obtained in January 2021 (OISO-31 cruise) and May 2022 (RESILIENCE cruise). To evaluate the decadal trends and drivers of fugacity of CO₂ (fCO₂), air-sea CO₂ fluxes, dissolved inorganic carbon (C_T) and pH, we used available data in this region over 1963–2023 and compared the results in the Mozambique Basin and in the Agulhas region near the African coast. Over 1995–2023, we found a faster fCO₂ increase in the Mozambique basin (2.03 ± 0.07 μatm.yr⁻¹) compared to the coastal zone (1.37 ± 0.07 μatm.yr⁻¹). The temporal change of anthropogenic CO₂ concentrations estimated in subsurface enables to reconstruct the carbonate system properties since the 1960s. In the Mozambique Basin the CO₂ sink increased slightly over 1960–2022 with a maximum observed in May 2022 (−2.4 mmolC.m⁻².d⁻¹). In the coastal zone, the ocean CO₂ sink increased from near equilibrium in the 1960s to a maximum observed in May 2022 (−4.2 mmolC.m⁻².d⁻¹). In both regions, we found a decrease of pH, most pronounced in the open ocean zone (−0.020 ± 0.001.decade⁻¹ over 1995–2023). The lowest pH of 8.04 was observed in January 2021, 0.11 lower than in the 1960s. The increase of the CO₂ sink and the decrease of pH were mainly driven by anthropogenic CO₂ uptake, with about 10% due to the ocean warming.